BOSTON – Testimony opened Monday in U.S. District Court in the trial of a former Plymouth police sergeant accused of assaulting a defenseless prisoner in a police holding cell.

But from the opening statements from prosecutors and the defense, it was apparent the case against Shawn Coughlin will also take a close look at the alleged victim of the attack, a drunken driving offender with a history of violence against police.

Prosecutors painted Coughlin, an 18-year veteran officer, as a "bully with a badge" who abused his powers in the fall of 2011 in a beat down of John Leighton Jr.

But the defense insisted Coughlin was only coming to the aid of a fellow officer when he tussled with the handcuffed prisoner he had booked only a year earlier for assaulting a different officer.

The trial before Judge Nathaniel Gorton hinges on a videotape of the incident that Plymouth police automatically recorded in the early hours of Nov. 19, 2011.

Assistant U.S. Attorney Ted Merrill suggested in his opening statement that Coughlin repeatedly hit Leighton in the head with the heel of his hand and then kneed him in the back as the West Plymouth man lay on the floor of his holding cell with his hands cuffed behind his back.

Defense Attorney Tom Drechsler maintained that Coughlin thought he was protecting probationary officer Jonathan Yule when he charged into the cell. The video, Drechseler said, will show that Leighton kicked at both officers, and there is no dispute that throughout the fracas Coughlin kept yelling at Leighton to stop kicking.

A jury of eight women and six men will see, but not hear, for themselves Tuesday morning when the trial resumes for its second day of evidence. The videotape, introduced by Ryan Ruggiero, systems administrator for the Plymouth Police Department and the trial's first witness, depicts images from above the holding cell but has no sound.

Coughlin, 46, is accused of violating Leighton's civil rights against undue use of force and then falsifying a record in a criminal investigation.

He and Yule allegedly wrote reports after the incident alleging that Leighton was the aggressor, assaulting them with boots.

Leighton, who was on probation at the time, served 48 days in the county jail before his attorney obtained a copy of the videotape and showed it to prosecutors.

Both Coughlin and Yule were placed on paid administrative leave after the police department learned of the incident.

Yule was never criminally charged. He eventually resigned to pursue other opportunities. Coughlin was eventually terminated from the police force but he has appealed the firing and can still take the case to an arbitrator.

Yule is slated to testify for prosecutors Tuesday, along with three other police officers in the station at the time of the incident.

Page 2 of 2 - The defense contends that Yule's testimony is tainted because he was still hoping to keep his job when he first testified about the incident to federal grand jurors.

Yule is one of 11 past or present Plymouth police officers named on the list of potential witnesses. Two District Court prosecutors and a judge are on the witness list as well.

Comparisons between the defendant and alleged victim were readily evident throughout the early stages of the trial.

In his opening statement, Dreschler recognized Coughlin's wife and three children sitting in the front row and contrasted Coughlin's education and experience with Leighton's propensity for violence.

Drechsler said evidence will show that just a year earlier, Coughlin booked Leighton for assault and battery on a police officer. And when Yule, a former classmate of Leighton's, stopped the alleged victim that night it was as a repeat drunken driving suspect.

Merrill cautioned jurors to stay focused on the charges and not be distracted by the victim. Leighton, he said, may never even testify.

"This case is not a referendum on whether John Leighton deserved to be roughed up. This is a case about a defendant, Shawn Coughlin, who crossed the line," Merritt said.